Electrically Benign Behavior of Grain Boundaries in Polycrystalline ${\mathrm{CuInSe}}_2$ Films

The classic grain-boundary (GB) model concludes that GBs in polycrystalline semiconductors create deep levels that are extremely harmful to optoelectronic applications. However, our first-principles density-functional theory calculations reveal that, surprisingly, GBs in ${\mathrm{CuInSe}}_2$ (CIS) do not follow the classic GB model: GBs in CIS do not create deep levels due to the large atomic relaxation in GB regions. Thus, unlike the classic GB model, GBs in CIS are electrically benign, which explains the long-standing puzzling fact that polycrystalline CIS solar cells with remarkable efficiency can be achieved without deliberate GB passivation. This benign electrical character of GBs in CIS is confirmed by our scanning Kelvin probe microscopy measurements on $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ chalcopyrite films.

Figure 1

Atomic structure of GBs containing dislocation cores adopted from GB structures determined in CdTe with (a) Se dangling bonds and (b) cation dangling bonds.

Figure 2

Calculated PDOS of (a) Cu, (b) In, and (c) Se atoms around the dislocation core in the relaxed GB shown in Fig. 1a.

Figure 3

Calculated PDOS of (a) Cu, (b) In, and (c) Se atoms around the dislocation core in the GB shown in Fig. 1a without atomic relaxation in the GB region.

Figure 4

Topography images of surfaces of the CIGS films grown on Mo-coated soda-lime glass (a) and Mo-coated boron silicate glass (c) and their corresponding SKPM images (b),(d) with the same gray scale of 200 meV. (e) Potential line profiles measured along the dotted lines shown in (b) (upper curve) and (d) (lower curve).